Method of cold forming a piece of sheet metal by bending or press moulding

ABSTRACT

A method of cold forming a piece of sheet metal ( 1 ) by bending or press moulding, in which method:—the piece of sheet metal is bent over a bevelled edge ( 12, 13 ) of a tool ( 10 ) so that the piece of sheet metal in the bending area (B 1,  B 2 ) is subjected to compressive stress, on the inner side and tensile stress on the outer side of the neutral layer. A gap is left between the sheet metal and an edge surface ( 14, 15 ) of the bevelled edge.—compressive force is then exerted on the piece of sheet metal outside the bending area, which is compressed transversally to the longitudinal direction of the bend, whereupon compressive force is exerted on the outer side of the bending area so that the bending area is subjected to tensile stress on the inner side and compressive stress on the outer side of the neutral layer.

FIELD OF THE INVENTION AND PRIOR ART

The present invention relates to a method of cold forming a piece of sheet metal by bending or press moulding.

In order to achieve a permanent deformation of a piece of sheet metal by bending or press moulding, the piece of sheet metal has to be subjected to such a force that the yield point of the material is reached in the parts of the piece of sheet metal that are bent in connection with the bending/press moulding operation. When a piece of sheet metal is bent, the piece of sheet metal will be subjected to compressive stress at the inner side of the bend and tensile stress at the outer side of the bend. Between the area with compressive stress and the area with tensile stress inside the piece of sheet metal, there is a layer, the so-called neutral layer, which constitutes a boundary between these areas. There is compressive stress in the sheet metal material on one side of the neutral layer and tensile stress in the sheet metal material on the opposite side of the neutral layer. When a piece of sheet metal is subjected to pure bending with such a force that the yield point of the sheet metal material is reached in the bending area, a first plastic zone in which the sheet metal material has compressive stress that has reached the yield point will be developed at the inner side of the bend, whereas a second plastic zone in which the sheet metal material has tensile stress that has reached the yield point will be developed at the outer side of the bend. The sheet metal material is plasticized in these plastic zones. The neutral layer and an elastic zone in which the sheet metal material is not plasticized are located between these plastic zones. The sheet metal material in a first part of the elastic zone, between the neutral layer and the first plastic zone, has compressive stress lower than the yield point, whereas the sheet metal material in a second part of the elastic zone, between the neutral layer and the second plastic zone, has tensile stress lower than the yield point. When the bending force on the piece of sheet metal is released, the compressive stress in said first part of the elastic zone will be released and strive towards a dilation of the piece of sheet metal transversally to the longitudinal direction of the bend and thereby act for a straightening of the bend, i.e. an increase of the bend radius. In the corresponding manner, the tensile stress in said second part of the elastic zone will be released and strive towards a contraction of the piece of sheet metal transversally to the longitudinal direction of the bend and thereby act for at straightening of the bend. Furthermore, when the bending force on the piece of sheet metal is released, the material at the neutral layer, which essentially has no tensile stress or compressive stress, strives to spring back to the original shape that this material had before being subjected to the bending force. Under the effect of the spring-back tendency of the last-mentioned material and the released stresses in the elastic zone, there will consequently be a certain straightening of the bend and a return of the piece of sheet metal in the direction towards its shape before the bending operation, i.e. the piece of sheet metal will spring back.

A high-strength metal material has a higher yield point than a softer metal material, which implies that the above-described problem with spring-back is more severe when it comes to bending or press moulding of a high-strength metal material as compared to a corresponding bending or press moulding of a softer metal material.

The desired final shape of a piece of sheet metal after bending or press moulding may for instance be achieved by bending the piece of sheet metal during the bending/press moulding operation so far that the piece of sheet metal will spring back to the desired final shape after being released from the forming tool.

However, it is very difficult to determine how a piece of sheet metal is to be bent in order to assume a desired final shape after spring-back, and extensive and time-consuming trials are therefore normally required before the correct forming geometry is achieved. Furthermore, even very small property variations between apparently identical pieces of sheet metal may cause a different spring-back and consequently result in varying final shapes of the pieces of sheet metal after bending/press moulding thereof. Furthermore, it is not possible to bend a piece of sheet metal by a bending angle larger than 90 degrees when using a conventional forming machine with forming tools that are linearly movable in relation to each other in mutually parallel directions, which entails that the largest bending angle after spring-back will be smaller than 90 degrees. In order to achieve a final bending angle of 90 degrees after spring-back in a piece of sheet metal by means of such a forming machine, the piece of sheet metal has to be formed in several steps, which may be complicated and time-consuming.

Another alternative is to counteract said spring-back by more or less completely plasticizing the material in the bending area of the piece of sheet metal and consequently eliminate the above-mentioned elastic zone. This may for instance be achieved by exerting such a powerful compressive action against the surface of the piece of sheet metal in the bending area that the yield point is reached through the entire piece of sheet metal. However, in order to achieve such an extensive plasticizing when forming pieces of sheet metal of high-strength metal materials with a high yield point, very high compressive forces are required, which may be difficult and sometimes impossible to achieve with a conventional forming machine.

OBJECT OF THE INVENTION

The object of the present invention is to achieve a new and favourable solution to the above-described problem with spring-back in connection with the cold forming of a piece of sheet metal.

SUMMARY OF THE INVENTION

According to the present invention, said object is achieved by means of a method having the features defined in claim 1.

According to the invention, the piece of sheet metal is bent over a bevelled edge of a tool so that the piece of sheet metal in the area of the bend thus formed, which area here being denominating bending area, is subjected to compressive stress on the side of the neutral layer facing the inner side of the bending area and tensile stress on the side of the neutral layer facing the outer side of the bending area, a gap being left between the piece of sheet metal and an edge surface of the bevelled edge on the inner side of the bending area. A compressive force is thereafter exerted on the piece of sheet metal outside said bending area in such a manner that the bending area is subjected to compression in the extension direction of the piece of sheet metal transversally to the longitudinal direction of the bend. After this compression, compressive force is exerted against the outer side of the bending area so that the bending area is pressed inwards towards said edge surface so that the bending area is subjected to tensile stress on the side of the neutral layer facing the inner side of the bending area and compressive stress on the side of the neutral layer facing the outer side of the bending area.

According to the invention, the bending area is consequently influenced in such a manner that the zones with compressive stress and tensile stress that are initially developed in the bending area in connection with the bending of the piece of sheet metal are shifted so that tensile stress instead of compressive stress is developed at the inner side of the bend and compressive stress instead of tensile stress is developed at the outer side of the bend. When the bending force on the piece of sheet metal is released, the compressive stress in the elastic zone on the side of the neutral layer facing the outer side of the bend will be released and strive towards a dilation of the piece of sheet metal transversally to the longitudinal direction of the bend and thereby act for a decrease of the bend radius. In the corresponding manner, the tensile stress in the elastic zone on the side of the neutral layer facing the inner side of the bend will be released and strive towards a contraction of the piece of sheet metal transversally to the longitudinal direction of the bend and thereby act for a decrease of the bend radius. In this case, the released stresses in the elastic zone will consequently counteract the previously mentioned spring-back tendency of the material at the neutral layer, which implies that the spring-back of the piece of sheet metal can be reduced and even completely eliminated in dependence on the relationship between the bend radius decreasing effect of the released stresses in the elastic zone and the bend radius increasing spring-back effect of the material at the neutral layer.

Other favourable features of the method according to the invention will appear from the dependent claims and the description following below.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will in the following be more closely described by means of embodiment examples, with reference to the appended drawings. It is shown in:

FIGS. 1 a-1 d schematic illustrations of different steps in a method according to an embodiment of the present invention of cold forming a piece of sheet metal, and

FIGS. 2 b-2 d schematic diagrams over the distribution of the stresses, at the different steps according to FIGS. 1 b-1 d, in a cut through one of the bending areas of the piece of sheet metal.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

Different steps in a method according to an embodiment of the present invention of cold forming a piece of sheet metal 1 of metallic material are schematically illustrated in FIGS. 1 a-1 d. In the illustrated example, the piece of sheet metal 1 is bent into U-shape by bending along two rectilinear and mutually parallel bending lines. However, the method according to the invention can also be used for bending a piece of sheet metal along one or more curved bending lines by press moulding.

The piece of sheet metal 1 may for instance be of high-strength metallic material, such as steel or aluminium.

The piece of sheet metal 1 is cold formed by means of a forming machine 2. In the illustrated example, the forming machine 2 comprises a first tool 10, over which the piece of sheet metal 1 is intended to be bent, and a second tool 20 which is arranged opposite the first tool 10. The first tool 10 has a support surface 11 facing a corresponding support surface 21 on the second tool 20. The first and second tools 10, 20 are mutually movable in the direction towards each other so as to allow a piece of sheet metal 1 received in the space between the tools to be clamped between the support surface 11 on the first tool and the support surface 21 on the second tool. The first tool 10 is provided with a first bevelled edge 12 in order to form a first bending line in the piece of sheet metal to be formed, and a second bevelled edge 13 in order to form a second bending line in the piece of sheet metal to be formed. These bevelled edges 12, 13 extend in parallel with each other on either side of the support surface 11. The respective bevelled edge 12, 13 has an incline edge surface 14, 15 extending between the support surface 11 and an adjacent lateral wall 16, 17 of the first tool 10. The transition between the respective lateral wall 16, 17 and the adjacent edge surface 14, 15 is rounded and the transition between the respective edge surface 14, 15 and the support surface 11 is also rounded.

The forming machine 2 also comprises a third tool 30, which comprises a first part 30 a located on a first side of the second tool 20 and a second part 30 b located on the other side of the second tool 20 opposite the first part 30 a. These first and second parts 30 a, 30 b are consequently located on either side of the second tool 20. The third tool 30 is linearly displaceable in relation to the first tool 10 and in relation to the second tool 20.

The piece of sheet metal 1 to be formed is placed with a central first part 1A of the piece of sheet metal bearing against the support surface 11 on the first tool 10 and with lateral second and third parts 1B, 1C of the piece of sheet metal extending out over the bevelled edges 12, 13 of the first tool 10 on either side of the first part 1A of the piece of sheet metal, as illustrated in FIG. 1 a. Consequently, said first part 1A of the piece of sheet metal is at a first side connected to a second part 1B of the piece of sheet metal that extends out over the first bevelled edge 12 of the first tool and is at an opposite second side connected to a third part 1C of the piece of sheet metal that extends out over the second bevelled edge 13 of the first tool. The first part 1A of the piece of sheet metal is received between the support surface 11 on the first tool and the support surface 21 on the second tool.

In the illustrated example, said support surfaces 11, 21 on the first and second tools are straight.

When the piece of sheet metal 1 has been placed between the first tool 10 and the second tool 20 in the above-mentioned manner, the first tool 10 and the third tool 30 are subjected to a mutual displacement in the direction towards each other so that the second part 1B of the piece of sheet metal is bent over the first bevelled edge 12 of the first tool 10 under the effect of the first part 30 a of the third tool 30 and thereby bent in relation to the first part 1A of the piece of sheet metal to form a first bending area B1 on the piece of sheet metal, and so that the third part 1C of the piece of sheet metal is bent over the second bevelled edge 13 of the first tool 10 under the effect of the second part 30 b of the third tool 30 and thereby bent in relation to the first part 1A of the piece of sheet metal to form a second bending area B2 on the piece of sheet metal, as illustrated in FIG. 1 b. During this mutual displacement between the first and third tools 10, 30, the first and second tools 10, 20 are kept at a distance from each other so that the first part 1A of the piece of sheet metal, in connection with the bending of the second and third parts 1B, 1C of the piece of sheet metal, is bulged outwards from the support surface 11 on the first tool into contact with the support surface 21 on the second tool. At this stage of the forming process, the piece of sheet metal 1 consequently extends in a bow between the two bevelled edges 12, 13 of the first tool, a gap provided between the first part 1A of the piece of sheet metal and the support surface 11 on the first tool. In connection with this initial bending of the piece of sheet metal, compressive stress is developed in the respective bending area B1, B2 on the side of the neutral layer 8 facing the inner side 3 of the bending area and tensile stress is developed in the respective bending area B1, B2 on the side of the neutral layer 8 facing the outer side 4 of the bending area, as illustrated in FIG. 2 b.

Tensile stress is indicated with a dash-patterned area 5 and tensile stress is indicated with a dot-patterned area 6 in FIGS. 2 b-2 d. Furthermore, the centre plane 7 of the piece of sheet metal is indicated with a broken line in FIGS. 2 b-2 d.

In connection with said initial bending of the piece of sheet metal, a gap is left between the piece of sheet metal 1 and the edge surface 14 of the first bevelled edge 12 on the inner side 3 of the first bending area B1 and a gap is left between the piece of sheet metal and the first part 30 a of the third tool on the outer side 4 of the first bending area B1. In the corresponding manner, a gap is left between the piece of sheet metal 1 and the edge surface 15 of the second bevelled edge 13 on the inner side 3 of the second bending area B2 and a gap is left between the piece of sheet metal and the second part 30 b of the third tool on the outer side 4 of the second bending area B2.

The first and second tools 10, 20 are then subjected to a mutual displacement in the direction towards each other so that the first part 1A of the piece of sheet metal is flattened between the support surfaces 11, 20 on these tools, while the third tool 30 retains the second and third parts 1B, 1C of the piece of sheet metal in the bent state by clamping these parts of the piece of sheet metal between the first tool 10 and said first and second parts 30 a, 30 b of the third tool 30, as illustrated in FIG. 1 c. Hereby, the above-mentioned bulge of the first part 1A of the piece of sheet metal is flattened so that the piece of sheet metal 1 in the respective bending area B1, B2 is subjected to compression in the extension direction of the piece of sheet metal transversally to the longitudinal direction of the bend. Material is pressed from the first part 1A of the piece of sheet metal into the respective bending area B1, B2 by this flattening of the bulge of the first part 1A of the piece of sheet metal. After this flattening of the first part 1A of the piece of sheet metal, the piece of sheet metal 1 extends in a first bow over the edge surface 14 of the first bevelled edge 12 at the first bending area B1 and in a second bow over the edge surface 15 of the second bevelled edge 13 at the second bending area B2, while leaving a gap between the piece of sheet metal and the edge surface 14 of the first bevelled edge 12 on the inner side 3 of the first bending area B1 and a gap between the piece of sheet metal and the edge surface 15 of the second bevelled edge 13 on the inner side 3 of the second bending area B2.

The first and second tools 10, 30 are thereafter subjected to a continued mutual displacement in the direction towards each other, while the first part 1A of the piece of sheet metal is retained in the flattened state between the first and second tools 10, 20, so that the respective bending area 61, B2 on the piece of sheet metal is pressed inwards by the third tool 30 against the adjacent edge surface 14, 15 of the first tool, as illustrated in FIG. 1 d, so that tensile stress is developed in the respective bending area B1, B2 on the side of the neutral layer 8 facing the inner side 3 of the bending area and compressive stress is developed in the respective bending area on the side of the neutral layer 8 facing the outer side 4 of the bending area, as illustrated in FIG. 2 d. During this stage of the forming process, the piece of sheet metal is in the first bending area B1 pressed in the direction towards the edge surface 14 of the first bevelled edge 12 under the effect of a shoulder 31 a arranged on the first part 30 a of the third tool, whereas the piece of sheet metal in the second bending area B2 is pressed in the direction towards the edge surface 15 of the second bevelled edge 13 under the effect of a shoulder 31 b arranged on the second part 30 b of the third tool. In the illustrated example, the respective shoulder 31 a, 31 b is inclined with an inclination corresponding to the inclination of the underlying edge surface 14, 15 of the first tool 10.

The formed piece of sheet metal 1 is finally released from the tools 10, 20, 30.

The above-mentioned mutual displacements between the first and third tools 10, 30 may for instance be achieved by keeping the first tool 10 stationary while the third tool 30 is displaced in the direction towards the first tool, or alternatively by keeping the third tool 30 stationary while the first tool 10 is displaced in the direction towards the third tool.

The above-mentioned mutual displacement between the first and second tools 10, 20 may for instance be achieved by keeping the first tool 10 stationary while the second tool 20 is displaced in the direction towards the first tool, or alternatively by keeping the second tool 20 stationary while the first tool 10 is displaced in the direction towards the second tool.

The invention is of course not in any way limited to the embodiments described above. On the contrary, several possibilities to modifications thereof will be apparent to a person with ordinary skill in the art without thereby deviating from the basic idea of the invention as defined in the appended claims. 

1. A method of cold forming a piece of sheet metal (1) by bending or press moulding, in which method the piece of sheet metal (1) is bent over a bevelled edge (12, 13) of a tool (10) so that the piece of sheet metal in the area (B1, B2) of the bend thus formed, being denominated bending area, is subjected to compressive stress on the side of the neutral layer (8) facing the inner side (3) of the bending area and tensile stress on the side of the neutral layer (8) facing the outer side (4) of the bending area, a gap being left between the piece of sheet metal (1) and an edge surface (14,15) of the bevelled edge (12, 13) on the inner side (3) of the bending area; and compressive force is thereafter exerted on the piece of sheet metal (1) outside said bending area (B1, B2) in such a manner that the bending area is subjected to compression in the extension direction of the piece of sheet metal transversally to the longitudinal direction of the bend, whereupon compressive force is exerted against the outer side (4) of the bending area so that the bending area (B1, B2) is pressed inwards towards said edge surface (14, 15) so that the bending area is subjected to tensile stress on the side of the neutral layer (8) facing the inner side (3) of the bending area and compressive stress on the side of the neutral layer (8) facing the outer side (4) of the bending area.
 2. A method according to claim 1, wherein the method comprises the following consecutive steps: A) the piece of sheet metal (1) is placed with a first part (1A) of the piece of sheet metal received between a first tool (10) and a second tool (20), and with second and third parts (1B, 1C) of the piece of sheet metal extending out over bevelled edges (12, 13) of the first tool (10) on either side of the first part (1A) of the piece of sheet metal; B) the first tool (10) and the third tool (30) are thereafter subjected to a mutual displacement in the direction towards each other so that the second and third parts (1B, 1C) of the piece of sheet metal are bent over the bevelled edges (12, 13) of the first tool (10) under the effect of the third tool (30) and thereby are bent in relation to the first part (1A) of the piece of sheet metal to form a first bending area (B1) on the piece of sheet metal at the first bevelled edge (12) and a second bending area (B2) on the piece of sheet metal at the second bevelled edge (13), while the first and second tools (10, 20) are kept at a distance from each other so that the first part (1A) of the piece of sheet metal is bulged in connection with this bending of the second and third parts (1B, 1C) of the piece of sheet metal, a gap being left in the respective bending area (B1, B2) between the piece of sheet metal (1) and an edge surface (14, 15) of the bevelled edge (12, 13) on the inner side (3) of the bending area and between the piece of sheet metal (1) and the third tool (30) on the outer side (4) of the bending area; C) the first and second tool (10, 20) are thereafter subjected to a mutual displacement in the direction towards each other so that the first part (1A) of the piece of sheet metal is flattened between these tools while the third tool (30) retains the second and third parts (1B, 1C) of the piece of sheet metal in the bent state, whereby the bulge formed in step B in the first part (1A) of the piece of sheet metal is flattened so that the piece of sheet metal in the respective bending area (B1, B2) is subjected to compression in the extension direction of the piece of sheet metal transversally to the longitudinal direction of the bend; D) the first and third tools (10, 30) are thereafter subjected to a continued mutual displacement towards each other, while the first part (1A) of the piece of sheet metal is retained in the flattened state between the first and second tools (10, 20), so that the respective bending area (B1, B2) of the piece of sheet metal is pressed inwards by the third tool (30) towards the adjacent edge surface (14, 15) of the first tool so that the respective bending area (B1, B2) is subjected to tensile stress on the side of the neutral layer (8) facing the inner side (3) of the bending area and compressive stress on the side of the neutral layer (8) facing the outer side (4) of the bending area; and E) the thus formed piece of sheet metal (1) is thereafter released from said tools (10, 20, 30).
 3. A method according to claim 2, wherein the first tool (10) is kept stationary in steps B and D, while the third tool (30) is displaced towards the first tool.
 4. A method according to claim 2, wherein the first tool (10) is kept stationary in step C, while the second tool (20) is displaced towards the first tool.
 5. A method to claim 1, wherein the piece of sheet metal (1) by bending is bent along two rectilinear and mutually parallel bending lines.
 6. A method according to claim 5, wherein the piece of sheet metal (1) is bent into U-shape.
 7. A method according to claim 1, wherein the piece of sheet metal (1) by press moulding is bent along one or more curved bending lines.
 8. A method according to claim 3, wherein the first tool (10) is kept stationary in step C, while the second tool (20) is displaced towards the first tool.
 9. A method according to claim 8, wherein the piece of sheet metal (1) by bending is bent along two rectilinear and mutually parallel bending lines.
 10. A method according to claim 2, wherein the piece of sheet metal (1) by bending is bent along two rectilinear and mutually parallel bending lines.
 11. A method according to claim 3, wherein the piece of sheet metal (1) by bending is bent along two rectilinear and mutually parallel bending lines.
 12. A method according to claim 4, wherein the piece of sheet metal (1) by bending is bent along two rectilinear and mutually parallel bending lines.
 13. A method according to claim 12, wherein the piece of sheet metal (1) is bent into U-shape.
 14. A method according to claim 11, wherein the piece of sheet metal (1) is bent into U-shape.
 15. A method according to claim 10, wherein the piece of sheet metal (1) is bent into U-shape.
 16. A method according to claim 9, wherein the piece of sheet metal (1) is bent into U-shape.
 17. A method according to claim 2, wherein the piece of sheet metal (1) by press moulding is bent along one or more curved bending lines.
 18. A method according to claim 3, wherein the piece of sheet metal (1) by press moulding is bent along one or more curved bending lines.
 19. A method according to claim 4, wherein the piece of sheet metal (1) by press moulding is bent along one or more curved bending lines. 